There hasn't been a rock-star vanity project movie as atrocious as Mariah Carey's Glitter for many a moon. Searching for films that plumb comparable depths of excremence, I had to ransack my memory as far back as Motown boss Berry Gordy's Mahogany, a vacuous love letter to his then-paramour Diana Ross, whose thespian talents only just exceed those of Carey. Ross was notoriously difficult to please, so when Mahogany crashed and burned within a nanosecond of its 1975 release, boy, did the lady make Gordy sing the blues.

He was browbeaten into throwing more good money after bad with 1978's The Wiz, a Motown remake of The Wizard of Oz, which - and this almost beggars belief - was directed by no less august and pricey a personage than Sidney Lumet. That, too, was a conceptual, commercial and critical disaster, and as a movie star, Diana Ross vanished faster than Fatty Arbuckle.

It's therefore quite remarkable how the Ross Crossover Paradigm has endured as a model for recording artists hoping to break into Hollywood. The films she made - backed at every step by Gordy - now seem to be just a series of private indulgences made horribly public. Even if one nurses a residual fondness for the overrated Lady Sings the Blues, Ross's movie career amounts to an object-lesson in how not to establish yourself in movies.

And yet this is the path Carey treads in Glitter. It may explain why, as the only new release in America last weekend, it failed even to enter the box-office charts, netting a derisory $2.5m in receipts. It was evident from the trailers, and then from the caustic and coruscatingly negative reviews, that even without America's recent tragedies, Glitter would have faced insurmountable obstacles in recovering its budget. Carey's own rags-to-riches-to-rehab career provides the narrative basis for Glitter.

We have the poor girl with the golden voice (or that mindless seven-octave virtuosity untroubled by any scintilla of soulfulness). After growing up in an orphanage with other hot abandoned girls like Da Brat, Mariah hooks up with a wicked, manipulative industry bigwig (not, of course, based on Carey's ex-Svengali, Sony Music boss Tommy Mottola). But her talent must out and everything culminates with world tours, platinum discs and global renown. Unfortunately, Carey's vocal talents are not matched by her non-existent thespian abilities, a narrow repertoire of lowered voice and elevated eyebrows. It's a non-performance fit to rank with that of her stone-faced rival Whitney Houston - the neo-Diana Ross - in The Bodyguard.

It's always the biggest musicians and rock stars who make for the most abominable movie actors. If the harrowing example of Elvis's entire cinematic career isn't ample proof of that, then take a look at the filmed output of the Material Girl. Madonna, so charismatic as a cultural icon, just looks dead on the screen, yet she persists in imagining herself as a reincarnation of Marilyn Monroe (a better actor and a better singer). Her accumulated output amounts almost to an indictable crime against cinematic culture. Shanghai Surprise, Body of Evidence and The Next Best Thing - these are her equivalents of GI Blues, Roustabout and Clambake.

How can they keep indulging her? It's a size thing. The bigger the star, the more money they earn. And the more power they have, the greater the urge to throw their weight about, and the harder it becomes to stare them down. Some rock stars are just too big to be ignored, espe cially in these days when the movie, TV and recording industries often nestle under the same corporate umbrella, making the crossover seem that much easier (it's a short walk from Geffen to Dreamworks or from WEA to Warner Bros). If a multimillionaire rock star wants to make a movie, the people who need the next album from them will cater to their every whim. Can you imagine those bigwigs trying to face down the wrath of Courtney Love?

Obviously nobody said no to Neil Diamond when he announced his wish not simply to become a movie star, but also to remake the first talkie, The Jazz Singer, and better yet, to hire Laurence Olivier to play his dad. No one said no to Barbara Streisand when she elected to make the rock version of A Star Is Born in 1976 (a stain on the Bicentennial festivities).

The bigger the ego, the harder the fall, but also the more cash there is to act as a cushion and a comfort. Rock stars are like some stand-up comics when it comes to crossing over. Suddenly they find they have to share the stage with others, and being accustomed to solitude in the spotlight, they lack the training to indulge in much actorly give-and-take.

Just as Robin Williams was once the greatest white comic of his generation and is now the worst movie actor, so David Bowie's much-vaunted acting skills proved illusory in The Hunger and Merry Christmas, Mr Lawrence, and Mick Jagger was a charisma-vacuum in Ned Kelly and Freejack. Donald Cammell and Nicolas Roeg had the right idea with that pair in Performance and The Man Who Fell to Earth, in which Jagger and Bowie were both cast in roles about six inches away from their real-life musical personae. It's the best solution: if they can't act for beans, then don't give them the chance.

It's the small-fry musicians who turn out best, like Dwight Yoakam, Meat Loaf, LL Cool J or Tupac Shakur - with Mark Wahlberg as the benchmark exemplar of how to cross over with dignity. Compare these with such artistic tragedies as Bob Dylan's performance in Hearts of Fire or Prince in Under the Cherry Moon. Turn it off! Turn it off!